1. Field of the Invention
The present invention relates to a rear focus wide-angle lens system having a half angle-of-view exceeding 30°, which is used in a small, light-weight digital camera, etc. The present invention also relates to an electronic imaging device using such a rear focus wide-angle lens system.
2. Description of Related Art
In recent years digital cameras and digital video cameras which employ a solid-state imaging sensor (image sensor) such as a CCD, etc., have become commonplace, and such cameras are also being utilized in mobile phones and PDAs (personal digital assistants), etc. In order to attain higher picture quality, the size of the image sensor is unavoidably increased. However, if the size of the image sensor is increased, the size of the photographic optical system designed for such an image sensor also has to be increased, and it becomes difficult to correct aberrations occurred in the photographic optical system. Furthermore, the pixel size of an image sensor is ever decreasing, and higher definition has been progressed. Consequently, a higher optical quality is required for the photographic optical system.
On the other hand, in an optical system utilizing such an image sensor, a long back focal distance is required due to the need to accommodate a filter group between the optical system and the image sensor. Moreover, telecentricity, in which light rays exiting from the optical system are incident onto the image sensor at right-angles thereto (perpendicular thereto), is also required. Hence, if an attempt is made to satisfy the above-mentioned conditions, the optical system tends to become larger. Therefore miniaturization of the optical system is an important factor, even in the case of a single focal-point lens system which has a relatively greater potential for miniaturization thereof.
Furthermore, a single focal-point lens system has often employed an entire lens-system advancing mechanism for focusing. However, a light-weight focusing lens group is required due to the use of smaller motors for driving the focusing lens group and due to increased focusing speed. Therefore a mechanically simple and convenient system in which a portion of the lens system is moved upon focusing is desired.
Various retrofocus optical systems having five lens elements, i.e., a lens element having a negative refractive power (hereinafter, a negative lens element), a lens element having a positive refractive power (hereinafter, a positive lens element), a negative lens element, a positive lens element and another positive lens element, in this order from the object, have been proposed, as examples of wide-angle lens systems which are telecentric and have a simple structure (e.g., Japanese Patent Nos. 3,943,988 and 3,723,654, and Japanese Unexamined Patent Publication No. 2003-241084).
However, in the optical system taught in Japanese Patent No. 3,943,988, an F-number is only 3.9, which is slow (dark), and a half angle-of-view is 24° through 32°, which is not sufficient for wide-angle use, and, field curvature and astigmatism are relatively large with respect to the focal length. These features are not enough to attain higher resolution.
In Japanese Patent No. 3,723,654, the half angle-of-view is only 25°, which is insufficient for a wide-angle lens system.
In Japanese Unexamined Patent Publication No. 2003-241084, a front lens group is provided on the object side of a diaphragm and a rear lens group is provided on the image side of the diaphragm, and focusing is carried out by the rear lens group. However, no provision is taught for preventing aberration fluctuations when focusing is carried out for an object at a closer distance; and, the angle-of-view is narrow, and astigmatism largely occurs.
In a fast wide-angle lens system, coma and astigmatism tend to largely occur, so that it is difficult for a conventional wide-angle lens system to attain an increased size and higher density of the image sensor. Furthermore, there are only a few conventional wide-angle lens systems that are designed with consideration to optical quality when focusing at a closer distance and to reduction in weight of the focusing lens group.